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The distinction between an aquatic plant and a terrestrial plant is often blurred because of the tendency for many aquatic species to have both submersed and emersed forms and because many terrestrial plants are able to tolerate periodic submersion. There are relatively few obligate submersed aquatic plants, (i.e. species that cannot tolerate emersion for even relatively short periods) but some examples include members of Hydrocharitaceae and Cabombaceae, Ceratophyllum, and Aldrovanda and most macroalgae (e.g. Chara and Nitella). Most aquatic plants can, or prefer to, grow in the emersed form, and most only flower in that form. Many terrestrial plants can tolerate extended periods of inundation, and this is often part of the natural habitat of the plant where flooding is common. These plants (termed helophytes) tolerate extended periods of waterlogging around the roots and even complete submersion under flood waters. Growth rates of helophytes decrease significantly during these periods of complete submersion and if water levels do not recede the plant will ultimately decline and perish.[citation needed]